In the field of dyeing, methods for dyeing have recently underwent rationalization. For example, jet dyeing capable of processing a large quantity of fiber in one lot has been broadly employed for dyeing of polyester fiber with disperse dyes. Jet dyeing is generally carried out in a beam dyeing system, a cheese dyeing system, a package dyeing system, and the like.
According to these dyeing methods, a dye dispersion is forcedly circulated in a stationary dense layer of fiber formed by winding fiber so many times. Therefore, the dye particles dispersed in a dye bath to be used in these methods are required to have a smaller size and to exhibit more excellent dispersion stability than in the conventional dyeing methods. In other words, if the dye particles are too large, their passage through the fiber layer is inhibited so that the dye does not sufficiently penetrate deep into the inside of the layer, floculates resulting in uneven dyeing between the inner layer and the outer layer or adheres to the outer surface of the fiber layer resulting in reduction in color fastness, such as rubbing fastness. Thus, the dyestuff which can be applied to these dyeing methods should have satisfactory dispersibility in a dye bath and also undergo no reduction in dispersibility in a broad temperature range of from room temperature to a high temperature at which dyeing actually occurs.
In general, however, dye dispersibility in a dye bath is liable to decrease upon heating the bath to a high temperature. As a result, the dye particles flocculate and adhere onto the surface of a material to be dyed as a filter cake. In the case where a material to be dyed forms a dense layer as described above, the dye concentration differs between the outer layer portion and the inner layer portion, failing to obtain level dyeing.
On the other hand, with the recent demand for saving resources and energy, dyeing conditions are being made more severe by lowering a liquor ratio (dyed materials to dyeing liquor) of a dye bath, e.g., from 1:30 to 1:10; decreasing a ratio of dye cake to dispersing agent, e.g., from 1:3 to 1:1; reducing the dyeing time and rising the dyeing temperature, e.g., from 130.degree. C..times.1 hour to 135.degree. C..times.0.5 hour; and the like. Since such increased severity in any dyeing condition is disadvantageous to dispersion stability of dyes, not a few disperse dyes whose dispersion stability has been relatively sufficient for the conventional dyeing methods are not satisfactory when used in the latest rationalized dyeing methods with the above-described increased severity.
More specifically, benzothiazole type monoazo dye of red hue represented by formula (I) ##STR2## is a compound obtained by reacting a diazo component and a coupling component in a usual manner, and its chemical structure per se is disclosed in British Pat. No. 1,324,235. The monoazo dye of formula (I) is capable of uniformly dyeing polyester fiber when used under conventional mild dyeing conditions. However, when the monoazo dye of formula (I) is used for dyeing of polyester fiber under the above-described rationalized severe conditions, its dispersibility seriously reduces, making it very difficult to achieve level dyeing. Even if dyeing is conducted under mild conditions, the resulting dyed materials do not have sufficient color fastness to light and rubbing, leaving room for further improvements. Further, the monoazo dye of formula (I) involves another problem of affinity for various dyeing assistants. For example, it exhibits extremely poor dispersion stability at high temperatures in the presence of sodium sulfate, which leads to unlevel dyeing in systems combined with reactive dyes containing sodium sulfate for dyeing of polyester/cotton blended fabric. In case of using as mixed dyes, poor affinity for other dyes causes color variation or unlevel dyeing.
Furthermore, in recent years, the surface of the fabric after dyeing is often subjected to post-finishing for the purpose of feeling improvement, waterproofing, and static charge prevention, such as polyurethane finish, silicon finish, etc. During the post-finishing, there is a tendency that the dye adsorbed on the polyester fiber undergoes bleeding to cause reduction in wet fastness. Therefore, it has also been demanded to develop dyes which exhibit satisfactory wet fastness even after post-finish.
As a result of extensive studies, it has now been found that the benzothiazole type monoazo compound represented by formula (I) has at least two crystal modifications, one of which is not so satisfactory in dispersion stability under a high temperature dyeing condition, while the other exhibits very satisfactory dispersion stability even under high temperature and severe dyeing conditions and that the cake obtained through ordinary synthetic reactions has the former crystal modification. It has been further found that dispersion stability of a dye composition in a high temperature bath is not only decided by the size of dye particles but also greatly depends on the above-described crystal modification. Based on these findings, the inventors have reached the conclusion that a stable dispersion system of a dye bath can first be attained by using the compound having the novel and preferred one of the aforesaid two crystal modifications.
As a result of continued investigations on the above-described monoazo dye having a specific crystal modification, it has been furthermore found that in cases where the monoazo dye of formula (I) having a specific crystal modification is mixed with other dyes, its excellent effects can be manifested exclusively when mixed with monoazo dyes having a specific structure hereinafter described.